1. Field of the Invention
The present invention relates to variable valve timing control systems for internal combustion engines, and particularly to a variable valve timing control system for an internal combustion engine having a variable valve timing adjuster, which system is provided with a lock mechanism for locking the rotational phase (as will be called the "camshaft phase") of a camshaft with respect to the crankshaft at a lock position substantially intermediate of its adjustable range.
On the other hand, the invention relates to a variable valve timing control system for an internal combustion engine, in which at least one of the intake valve and the exhaust valve of the internal combustion engine is provided with a variable valve timing adjuster having a lock mechanism.
2. Related Art
In recent years, more and more internal combustion engines to be mounted on vehicles have adopted a variable valve timing control system with a view to improving the output power, lowering the fuel consumption and reducing the exhaust emission. In a basic construction of a vane type variable valve timing control system, for example, there are coaxially arranged a housing 1 made rotatable in synchronism with the crankshaft of the engine a rotor 2 connected to the camshaft of an intake (or exhaust) valve, as shown in FIG. 15, and fluid chambers 3 formed in the housing 1 are defined into advance angle chambers 5 and retard angle chambers 6 by vanes 4 provided on the rotor 2. Moreover, the oil pressures in the advance angle chambers 5 and the retard angle chambers 6 are controlled by a hydraulic control valve to change the rotational phase (or the camshaft phase) of the camshaft with respect to the crankshaft thereby to control the valve timings variably.
In the vane type variable valve timing control system of the related art, when the engine stops (or when the oil pressure falls), the relative rotations of the housing 1 and the rotor 2 (or the vanes 4) are locked by a lock pin 7 at the most retard angle phase of the camshaft phase (or the valve timing of the intake valve), so as to prevent the noise by the vibrations of the vanes 4 at the starting time. As a result, the start is made in the most retard angle phase so that the most retard angle phase is set to a phase suited for the start.
With this construction, however, the most retard angle phase is restricted by the phase (or the lock position) at the start so that the adjustable range of the valve timing (or the camshaft phase) is restricted by the lock position to raise a defect that the adjustable range of the valve timing is narrow.
As disclosed in Unexamined Published Japanese Patent Application No. 9-324613, therefore, it has been proposed to widen the adjustable range of the valve timing by setting the lock phase at the engine stop at a substantially intermediate position of the adjustable range of the valve timing. According to this construction, at the engine start, the valve timing is locked in the intermediate lock phase. As the oil pressure rises with the subsequent increase in the engine speed (or the oil pump speed), the oil pressures in the advance angle chambers 5 and the retard angle chambers 6 rise to push the lock pin 7 out of the lock hole thereby to unlock the lock pin 7. After this lock release, the variable valve timing control can be made so that the hydraulic control valve is subjected to a feedback control to adjust the real valve timing to the target valve timing.
However, the lock position is given individual differences (or dispersion) by the manufacture errors or the like. When whether or not the normal lock state is to be judged, therefore, the range for the normal judgment has to be widened to consider the manufacture dispersion of the lock position. As a result, a fixture at a position other than the lock position may be judged as the normal lock state, or the detection of the lock release during the lock release control may be delayed.
During the lock release control, on the other hand, the oil pressure in the lock releasing direction is applied to the lock pin 7 but is not always suited for the lock release due to the manufacture dispersions of the lock position or the hydraulic control valve. This raises a problem that the lock release is delayed to affect the control of the valve timing adversely.
Here, the lock pin 7 for locking the camshaft phase at the lock position is biased in the lock direction by a spring and is held at the lock release position by the oil pressure while the engine is running. This oil pressure is raised by the oil pump driven by the power of the engine. When the engine stops, therefore, the oil pressure falls so that the lock pin 7 is inserted into the lock hole by the spring to lock the camshaft phase at the lock position. In order that the lock pin 7 may come into the lock hole, however, these two have to be aligned. As disclosed in the Specification of Japanese Patent Application No. 11-222031, therefore, we have applied such an invention for patent that the oil pressure is controlled to move the camshaft phase to the lock position when the engine is to be stopped.
When the engine stops, however, the rotating speed of the oil pump goes down to lower the oil pressure. If the variable valve timing adjuster has a poor movement, it cannot bring the camshaft phase to the lock position till the engine stop and may not lock the camshaft phase. If the camshaft phase cannot be locked at the lock position when the engine stops, the valve timing (or the camshaft phase) cannot be controlled at the next starting time to the target value (in the vicinity of the lock position) before the engine speed (or the oil pump speed) goes up to raise the oil pressure. As a result, the start is made at a valve timing outside of the target value so that the startability is deteriorated to elongate the engine starting time. If the engine is started without the camshaft phase being locked, moreover, the position of the vanes 4 is not fixed till the oil pressure rises, thereby to cause a problem that the vanes 4 collide against the housing 1 to generate a noise.
Here, the variable valve timing adjuster employs the discharge oil pressure of the oil pump driven by the engine as its drive source so that the valve timing cannot be hydraulically held in a constant phase till the oil pressure rises to some level after the engine start. In the related art, therefore, the variable valve timing adjuster is provided with a lock mechanism, and the engine is started with the valve timing being mechanically locked at a constant phase by the lock mechanism. When the oil pressure rises to a level after the start, it is applied to the lock mechanism in the unlocking direction. After this, the valve timing is variably controlled by controlling the oil pressure.
Even with the lock release control, however, the lock mechanism cannot be unlocked for some cause. In this case, therefore, it is conceivable to repeat the lock release control. Then, the lock can be released by the repeated lock release control, if the lock release failure is caused temporarily because the lock mechanism bits a foreign substance. However, the cause for the lock release failure is thought to come not only from the failure of the lock mechanism but also from the failure of the hydraulic control valve. In the case of this failure of the hydraulic control valve, this valve will not operate normally even if a signal for the lock release control is outputted thereto. Therefore, the lock mechanism itself cannot be unlocked even if it is normal.
On the other hand, the lock pin 7 for locking the camshaft phase is biased in the locking direction by the spring, and the lock release is effected by applying the oil pressures of both the advance angle chambers 5 and the retard angle chambers 6 to the lock pin 7 in the lock releasing direction. During the engine stop, the oil pressure falls so that the lock pin 7 is inserted into the lock hole by the spring force to hold the camshaft phase in the locked state at the intermediate lock position. Therefore, the engine is started with the camshaft phase being locked at the intermediate lock position. As the oil pressure rises according to the subsequent increase in the engine speed (or the oil pump speed), the oil pressures in the advance angle chambers 5 and the retard angle chambers 6 rise to push the lock pin 7 out of the lock hole so that the lock pin 7 is unlocked. After this lock release, the valve timing control can be made so that the hydraulic control valve is feedback-controlled to adjust the real valve timing (or the real advance angle position) to the target valve timing (or the target advance angle position).
However, the construction of the related art does not have the function to detect the lock release of the lock pin 7. Therefore, the feedback control is started before the end of the lock release, and the hydraulic control valve is feedback-controlled to adjust the real advance angle position of the camshaft phase to the target advance angle position. Just after the start, the target advance angle position is usually set in the vicinity of the intermediate lock position, but frequently goes apart from the intermediate lock position as the time elapses. If the timing for the lock release is delayed for some cause, therefore, the deviation between the real advance angle position and the target advance angle position increases to make such a feedback control as to raise the oil pressure of one of the advance angle chambers 5 and the retard angle chambers 6 but lower the oil pressure of the other. The result is to enlarge the unbalance between the pressures of the two chambers 5 and 6. In this unbalanced oil pressure state, the lock pin 7 is strongly pushed by the oil pressure onto the inner side face of the lock hole so that it is bitten by the lock hole. This establishes a vicious cycle to retard the timing of the lock release more and more. As a result, the valve timing control is not made normally to cause problems to invite problems in the reduction of the drivability, the deterioration of the fuel economy, and the deterioration in the exhaust emission.
On the other hand, most variable valve timing systems now in practice advance the angle of the valve timing of the intake valve in dependence on the engine running state. In recent years, however, there has been developed a variable valve timing system which is provided the variable valve timing adjusters at both the intake valve and the exhaust valve so as to enhance the variable valve timing control performance. With this construction, when the lock mechanism for locking the valve timing fails operationally so that the variable valve timing control is made in an unlocked state, only the variable valve timing in the normal state operates as usual. As a result, the valve overlap of the intake and exhaust valves may become abnormal to make the exhaust residual ratio (or the internal EGR) in the cylinder of the engine. Then, the combustion state of the engine may be deteriorated to cause a misfire or to deteriorate the drivability or the exhaust emission.